1. Field of the Invention
The present invention relates to a high electron mobility transistor (HEMT) and a process of forming the HEMT.
2. Background Art
A Japanese Patent document published as JP-2006-261642A has disclosed a field effect transistor (FET) and a method of forming the same. FIG. 8 schematically illustrates a cross section of the FET disclosed therein. The FET 100 provides a substrate 101 made of sapphire, and semiconductor layers formed on the substrate 101, where the semiconductor layers include a buffer layer 102 made of undoped gallium nitride (GaN) that forms a channel layer of the FET, an electron supply layer 103 made of n-type aluminum gallium nitride (AlGaN) provided on the buffer layer 102, and a cap layer 104 made of n-type indium-aluminum-gallium nitride (InAlGaN) provided on the electron supply layer 103. The FET further provides, on the n-type InAlGaN cap layer 104, ohmic electrodes 105 made of stacked metal of titanium (Ti) and aluminum (Al), where those electrodes 105 are a source electrode and a drain electrode of the FET 100.
A portion of the n-type InAlGaN cap layer 105 exposes a surface of the n-type AlGaN electron supply layer 103. A gate electrode 106, which is a stacked metal of palladium (Pa) and silicon (Si), is provided on the exposed surface of the n-type AlGaN electron supply layer 103. The gate electrode 105 makes a Schottky contact against the n-type AlGaN electron supply layer 103.